Does a reference to another document which is no longer available or lost make a contract void or nullified?
Such as a service agreement between two companies that makes reference to a marketing budget established in Schedule A, And Schedule A is either missing or was never created.
No, that would not nullify the contract. This is not that uncommon of an occurrence so there is a lot of precedent for dealing with this kind of situation.
Generally, you don't look beyond the face of the contract to understand each party's obligations, but where a part of the contract is missing, you can use "extrinsic" evidence to help understand what the parties actually agreed to. For example, this could include drafts of the contracts or emails relating to the contract.
I agree with the first post. If the schedule was prepared but is just missing, then it's an issue, but it doesn't make the contract void. If the schedule was never prepared, and is it's fundamental to the contract, though, then that creates much more of a problem.
How this would play out in court would depend on the facts and circumstances of the situation.